1. Technical Field
The present invention relates to fluid flow rate measurement devices, and more particularly to an apparatus and a method for measuring the flow rate of a fluid whereby the propagation times of ultrasonic signals transmitted through the fluid vary with the flow rate of the fluid and can be detected to determine fluid flow rate.
2. Discussion
Transient-Time ultrasonic flow sensors, also known as time-of-flight ultrasonic flow sensors, detect the propagation time difference between the upstream and downstream ultrasonic transmissions resulting from the movement of the flowing fluid and process this information to derive fluid flow rate. These sensors typically use a pair of multiplexed transducers to permit each to alternately provide both transmit and receive functions. Relatively complex multiplexing and transit-time detection electronic circuitry is necessary for achieving usable measurement precision because the change in transit-time due to the flowing fluid is typically a very small part of the total transit time. The transducers are most often mounted on the outside of the pipe which affords the convenience of avoiding penetration of the pipe. However, the uncertainty of the pipe wall uniformity and surface condition, and the variabilities of locating and attaching the transducers, often constitute unfavorable conditions which can lead to substantial measurement error. These sensors are relatively complex and expensive, and have a reputation for sometimes producing erroneous readings.
It is therefore an object of the present invention to provide improved means for processing the signals, reducing the costs and improving the reliability of transit time flow sensors.
The present invention relates to an apparatus and a method for measuring the low rate of a flowing fluid through the use of ultrasonic energy transmitted through the flowing fluid, and by monitoring the shift in frequency of transmitted and received ultrasonic signals caused by the flowing fluid. One preferred embodiment of the present invention comprises an insertion probe whereby two flow sensing ultrasonic transducers are permanently mounted on the probe which enters the pipe carrying the flowing fluid. The transducers are located one upstream and one downstream, in line with and at an angle to the flowing fluid, and are directly wetted by the fluid. The transducers are also mounted and well insulated acoustically to minimize the transfer of acoustical energy between them which has not been shifted in time by the fluid flow. A probe-mounted acoustically reflective surface is also preferably provided to enable an acoustic path between the transducers to be completed.
In a basic form of the present invention, one transducer transmits a high frequency acoustic signal which passes through the flowing fluid to the reflector and then again through the flowing fluid to the receiving transducer. The transmitted and received signals are compared in a phase detector which provides an output signal controlling the frequency of the transmitted signal to maintain a constant phase difference between the two signals, as in a phase locked loop. The change in the transit time of the acoustic signal caused by a flowing fluid causes an incremental phase shift between the transmitted and received signals which is also detected by the phase detector. The phase detector output varies the frequency of the transmitted signal which in turn varies the wavelength of the acoustic signal and therefore the acoustic phase shift between the transducers, to minimize the change in detected phase shift. That change in frequency is representative of the fluid flow rate. Since the acoustic phase shifts due to fluid flow rate are typically on the same order of magnitude as those from error sources, such as transducer, mechanical and electronic drifts, the elementary form of the present invention may likely suffer from instability and might have limited utility.
In one preferred form of the present invention the receive and transmit functions of two transducers are interchanged at a low frequency rate. The transmitted frequency from a reference oscillator is compared against the frequency of a variable frequency slave oscillator. The slave oscillator frequency is controlled to be the same as that of the reference oscillator and to retain that frequency just prior to the reversal of the transducer functions. For most of the operating period in each mode, the slave oscillator frequency is that of the reference oscillator during the prior mode of operation. A fluid flow direction which in one mode of operation causes the reference oscillator to increase in frequency, results in the slave oscillator producing, during almost all of the same period, a corresponding reduction in oscillator frequency. Both oscillators alternately swing high and low in frequency out of phase tending to cancel their error contributions. The two frequencies are combined, whereby the difference frequency is detected and provided as an output signal indicative of flow rate. A relatively low frequency oscillator is also used to occasionally reset the reference oscillator to the nominal center of its operating range to correct for transducer mechanical or electronic related drifts over a period of time.
At zero fluid flow rate, the reference and slave oscillator frequencies are equal and the output frequency is therefore automatically zero. When fluid flow occurs, a continuous output frequency signal is produced representative of the flow rate. Since the output frequency range can easily be several kilohertz or even tens of kilohertz, a very wide dynamic range of operation is inherent with this electronic processing means. Furthermore, as phase detection is used in a feedback system over a narrow range, its error contribution to the fluid flow rate measurement error is small compared to that of a phase detector used to detect the full range of time difference between the transmitted and received signals. Such processing means are therefore applicable to a wide range of transit-time ultrasonic flow sensors and, in particular, to insertion probe and 2-measurement axis sensor configurations.